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exposed to air: laboratory-scale investigations using DGT-based fractionation
Andreina Laera, Rémy Buzier, Gilles Guibaud, Giovanni Esposito, Eric D.
van Hullebusch
To cite this version:
Andreina Laera, Rémy Buzier, Gilles Guibaud, Giovanni Esposito, Eric D. van Hullebusch. Dis- tribution trend of trace elements in digestate exposed to air: laboratory-scale investigations using DGT-based fractionation. Journal of Environmental Management, Elsevier, 2019, 238 (159-165),
�10.1016/j.jenvman.2019.02.120�. �hal-02053015�
Distribution trend of trace elements in digestate exposed to air: laboratory-scale investigations using DGT-based
fractionation
Authors and affiliation
Andreina Laeraa,b, Rémy Buzierb*, Gilles Guibaudb, Giovanni Espositoc, Eric D. van Hullebuschd
aUniversity of Paris-Est, Laboratoire Géomatériaux et Environnement (EA 4508), UPEM, 77454 Marne-la-Vallée, France, [email protected] or [email protected]
b University of Limoges, PEIRENE, Equipe Développement d’indicateurs ou prévision de la qualité des eaux, URA IRSTEA, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France
c University of Napoli “Federico II”, Department of Civil, Architectural and Environmental Engineering, via Claudio 21, 80125 Napoli, Italy
d Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154, CNRS, F-75005 Paris, France
*Corresponding author: [email protected]
Highlights
Distribution trend of 14 trace elements was studied in digestate under air exposure;
DGT was used as trace elements fractionation tool to assess the labile fraction;
Aeration promoted dissolution of Al, As, Co, Cr, Cu, Fe, Mn, Mo and Pb;
Forced aeration promoted an increase of labile Al, As, Co, Mo, Ni, Sb, Se and W;
Al, As, Co, Cr, Cu, Fe, Mn and Pb were mainly present as particulate despite aeration.
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62 1
Distribution trend of trace elements in digestate exposed to air: laboratory-scale investigations using DGT-based
fractionation
Authors and affiliation
Andreina Laeraa,b, Rémy Buzierb*, Gilles Guibaudb, Giovanni Espositoc, Eric D. van Hullebuschd
aUniversity of Paris-Est, Laboratoire Géomatériaux et Environnement (EA 4508), UPEM, 77454 Marne-la-Vallée, France, [email protected] or [email protected]
b University of Limoges, PEIRENE, Equipe Développement d’indicateurs ou prévision de la qualité des eaux, URA IRSTEA, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France
c University of Napoli “Federico II”, Department of Civil, Architectural and Environmental Engineering, via Claudio 21, 80125 Napoli, Italy
d Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154, CNRS, F-75005 Paris, France
*Corresponding author: [email protected]
Abstract
The use of digestate as amendment for agricultural soils has already been proposed as an 1
alternative to mineral fertilizers or undigested organic matter. However, little information is 2
available concerning the effect of digestate atmospheric exposure on trace elements speciation 3
and, consequently, on their mobility and bio-accessibility when digestate is stored in open tanks 4
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or handled before land spreading. In this study, we investigated at laboratory-scale the effect of 5
digestate aeration on the distribution of Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se and 6
W using the diffusive gradients in thin films technique (DGT)-based fractionation. For this 7
purpose, experiments were performed to assess the variation in distribution between the labile, 8
soluble and particulate fractions over time in digested sewage sludge during passive and forced 9
aeration. Results showed that aeration promoted a dissolution of Al, As, Co, Cr, Cu, Fe, Mn, Mo 10
and Pb, suggesting a possible increase in their mobility that may likely occur during storage in 11
open tanks or handling before land spreading. Labile elements’ fraction increased only during 12
forced aeration (except for Fe and Mn), suggesting that their short-term bio-accessibility can 13
increase only after significant aeration as the one assumed to occur when land spreading takes 14
place.
15
Keywords
Metals Metalloids
Digested sewage sludge Fractionation
Diffusive Gradients in Thin Films (DGT) Speciation
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62 3
1. Introduction
The use of digestate, a by-product of anaerobic digestion of organic residues (Möller and Müller, 16
2012), as amendment for agricultural soils and substitute of mineral fertilizers (Riva et al., 2016) 17
is gaining importance as a result of the increasing use of biogas plants running on different 18
organic feedstock (Scarlat et al., 2018). However, the presence of potentially hazardous trace 19
elements (TEs) (e.g. cadmium (Cd), copper (Cu) and lead (Pb)) in digestate, may prevent its use 20
in agriculture (Kupper et al., 2014; Tampio et al., 2016). The bio-accessibility of TEs not only 21
depends on their total concentration but also on their speciation (Hooda, 2010). Therefore, 22
screening of TEs speciation is required to assess the harm or benefit associated with digestate 23
before land spreading (van Hullebusch et al., 2016).
24
According to the spreading season, digestate could be stored for several months (Plana and 25
Noche, 2016) in open tanks (Boulamanti et al., 2013; Liebetrau et al., 2010). During storage in 26
open tanks and handling before land spreading, digestate will be exposed to air. Such exposure 27
will alter the anaerobic status of digestate which in turn may alter the speciation of TEs and 28
consequently affect their mobility and bio-accessibility in the environment. Although no 29
information is available, to the best of our knowledge, for digestate, Øygard et al. (2007) 30
demonstrated that atmospheric exposure impacts on TEs’ distribution in municipal solid waste 31
landfill leachates. Therefore, new investigations are needed to assess the potential impact of 32
digestate aeration on TEs speciation for risk assessment before land application.
33
Total element content in digestate is commonly determined after solubilization (usually acid 34
digestion) with conventional methods for TEs analysis in liquids such as ICP-MS (Dragicevic et 35
al., 2018a) and ICP-OES (Cao et al. 2018). The mobility and bio-accessibility of TEs in digestate 36
are usually studied using different techniques such as sequential extractions like the modified 37
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Tessier method (Ortner et al., 2014) or extraction with deionized water only (Dragicevic et al., 38
2018b). Alternatively, the diffusive gradients in thin films technique (DGT) can be used to screen 39
the presence of labile elements (i.e. the most readily bio-accessible form of TEs (Zhang and 40
Davison, 2015) into the environmental matrix. In particular, DGT-based fractionation was 41
recently validated for digestate matrix (Laera et al., 2019). Compared to conventional 42
fractionation techniques, DGT has the advantage of measuring the targeted elements in situ 43
without affecting the sample and speciation of TEs (Vrana et al., 2005). Moreover, DGT 44
technique increases the sensitivity of TEs monitoring compared to total acid-soluble 45
measurements (Laera et al., 2019).
46
Here, the effects of aeration of digested sewage sludge on mobility and bio-accessibility of 47
fourteen TEs were investigated to assess their fate before land spreading. The TEs investigated in 48
this study are either under EU regulation for application of sewage sludge in agriculture 49
(European Commission, 2016) (i.e. Cd, Cr, Cu, Ni and Pb), or selected based on previous studies 50
(Dragicevic et al., 2018b; Hamnér and Kirchmann, 2015; Laera et al., 2019; Øygard et al., 2007) 51
(i.e. Al, As, Co, Fe, Mn, Mo and Se). Antimony (Sb) and W were included because they could be 52
present in sewage sludge (Fu and Tabatabai, 1988; Healy et al., 2016; McBride, 2003) and 53
generate environmental issues due to their accumulation in plants (Arai, 2010; Charter et al., 54
1995).
55
In this study, the conventional particulate/soluble fractionation indicating potential TEs’ mobility 56
was implemented with a DGT-based fractionation procedure to monitor the most bio-accessible 57
species. Experiments were performed at laboratory-scale to assess the time variation of labile, 58
soluble and particulate TEs during passive and forced aeration of digestate. Results were 59
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62 5
discussed assuming that the experimental work can mimic digestate oxidation during storage in 60
open tanks or handling before land spreading.
61
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2. Material and methods
62
2.1. Digestate sample 63
Digested sewage sludge was collected from a municipal wastewater treatment plant located in 64
Limoges (France). The digestate derived from activated sludge treated by a mesophilic anaerobic 65
digestion process. About 18 L of sample was collected directly from a pipe before discharge in an 66
open storage tank. The sample was collected in a polypropylene (PP) bucket up to maximum 67
capacity and closed with a lid to limit sample exposure to open air. Once in the laboratory, the 68
sample was stored at 4°C for less than 24 hours before starting the experiment.
69
2.2. DGT preparation 70
We used Chelex-DGTs for cationic species (Al, Cd, Co, Cr (III), Cu, Fe, Mn, Ni and Pb) and 71
zirconia-DGTs (Zr-DGTs) for anionic species (As, Mo, Sb, Se and W). Each DGT consisted of a 72
binding gel (Chelex or Zr), a diffusive gel and a filter membrane enclosed in a piston type holder 73
(purchased from DGT Research, Lancaster, UK). Chelex binding gels were prepared according to 74
the procedure described by Zhang et al. (1998), whereas Zr binding gels were made according to 75
Devillers et al. (2016). Diffusive gels were standard polyacrylamide gels (15% acrylamide and 76
0.3% agarose-derived cross linker, 0.77 mm thick) prepared according to Zhang et al. (1998) and 77
filter membranes were made of cellulose acetate (0.2 µm pore size, 0.12 mm thickness, 78
Whatman, UK).
79
2.3. Experimental set-up 80
About 18 L of digested sludge were poured into a laboratory-scale PP tank placed under a fume 81
hood and continuously stirred with an overhead plastic propeller at 30 rpm (Figure S1) in order to 82
control experimental conditions. Stirring allows optimizing air transfer within the digestate and 83
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62 7
therefore represents a “worst case scenario” compared to unstirred real scale tanks. A Tinytag 84
data logger (TG-4100, Gemini Data Loggers, UK) was used to record the temperature in the 85
sample every 10 min. The surface of the sample was exposed to air to promote oxidation of the 86
sample during 10 weeks. The surface to volume ratio varied from 0.39 dm-1 (7.1 dm2:18 L) to 87
0.51 dm-1 (7.1 dm2:14 L) during the experiment because of multiple sample collection (see 88
below). Therefore, passive aeration was progressively favored while the experiment continued.
89
Then, aeration was enhanced during 2 supplementary weeks by introducing 4 micro-bubble air 90
diffusers in the digested sludge. All diffusers were connected to air pumps (Newair or Optima) 91
having airflow rates from 60 to 200 L/h.
92
Labile TEs were sampled by deploying three DGTs probes composed either of Chelex or Zr for 93
24h in the digested sludge. We choose a 24h deployment because it was shown previously to be a 94
good compromise for the studied elements in digestate (Laera et al., 2019).
95
DGTs were deployed according to the following sequence (Figure S1): every day for the 6 first 96
consecutive days; once per week from week 2 to 10; twice per week for weeks 11 and 12. Blanks 97
DGT devices were also prepared in duplicate and treated alongside exposed devices every week.
98
After DGTs’ retrieval, we measured dissolved O2, redox potential (Eh) and pH. Additionally, we 99
collected an aliquot of sample to measure total and volatile solids (TS and VS), total and volatile 100
suspended solids (TSS and VSS) and soluble TEs. Additionally, we monitored sulfate (SO42-) 101
concentration.
102
2.4. Analytical procedures 103
2.4.1. Physicochemical analysis 104
pH and Eh were measured with a Mettler Toledo pH meter and a Radiometer electrode, 105
respectively. Dissolved oxygen was measured using a ProODO™ optical sensor (YSI). Each 106
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sampling time, about 90 mL of sample was collected in duplicate to measure the total solids (TS), 107
volatile solids (VS), total suspended solids (TSS) and volatile suspended solids (VSS) according 108
to the French standard AFNOR NF T90-105 method.
109
The supernatant recovered during the TSS and VSS analysis was conserved to determine soluble 110
TEs (see section 2.4.2.).
111
2.4.2. Sample treatment and trace elements analysis 112
Total elements’ content was determined at the beginning and at the end of the experiment using 5 113
g of raw sample. Each sampling time, soluble elements’ concentration was determined from the 114
supernatant recovered during TSS determination. Supernatants and raw samples in duplicate were 115
acid digested with 6 mL of 69% HNO3 and 3 mL of 37% HCl in a microwave oven (Multiwave 116
GO, Anton Paar GmbH) at 180°C for 60 min.
117
TEs were analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700×) 118
except for Fe which was analyzed by microwave plasma atomic emission spectroscopy (MP- 119
AES, Agilent 4210). Blanks and quality controls at 5 and 10 µg/L were analyzed every 10 120
samples. The recovery was equal or above 86% for each element, except for Sb and W which was 121
equal or above 79% and 76%, respectively, among all analyses.
122
2.5. Element’s fractionations calculation 123
The fractionation procedure is presented in Figure 1. Particulate elements’ concentration was 124
calculated by subtracting the soluble to the initial total elements content.
125
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62 9
126
Figure 1. Fractionation procedure adopted in this study to estimate total, soluble, particulate and labile elements’ fractions.
127
After retrieval from the digested sludge, DGT samplers were rinsed with ultrapure water and 128
disassembled to recover the binding gels and determine labile elements concentration. The 129
accumulated mass (m) was determined following elution of binding gels in 2 mL of 1 M HNO3
130
or 5×10-3 M NaOH and 0.5 M H2O2 for 24 hours for Chelex and Zr-binding gels, respectively 131
(see Table S1 for elution yields). The concentration of labile TEs, CDGT, was then derived using 132
equation (1) based on Fick’s first law (Zhang and Davison, 1995):
133
, Eq. (1) 134
where ΔMDL is the thickness of the material diffusion layer (i.e. diffusive gel plus membrane, 0.89 135
mm), t is the time of DGT samplers’ exposure in the sludge (24h), D is the coefficient of 136
diffusion of the considered element and A is the geometric area of the DGT holder window (3.14 137
cm2).D values were taken from literature (Table S2) and corrected for the average temperature 138
recorded during each deployment using Stokes–Einstein relation (Zhang and Davison, 1999).
139
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The method’s limits of detection and quantification (namely MLD and MLQ for total and soluble 140
elements or MLDDGT and MLQDGT for labile elements) are displayed in Table S3 and S4.
141
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62 11
3. Results and discussion
142
3.1. Sample characterization 143
The characteristic of the digested sewage sludge (TS, VS and water concentration) are presented 144
in Figure S2. The results show that the water concentration and the VS content is nearly constant 145
throughout the experiment. In particular, the average water content was 96.2% ± 1.6 and the 146
average VS content was 63.9% ± 1.3. Moreover, the average pH of the digested sludge was 7.8 ± 147
0.3 and the Eh was below -50 mV, whatever the aeration of the sludge. The latter is shown in 148
Figure S3.
149
The total elements concentration in the digested sludge is shown in Table S5. Except for Cd, Mo 150
and Ni, the concentration of total elements is not significantly different (p>0.05) at the beginning 151
and at the end of the experiment. For total Cd, Mo and Ni content the difference was significant 152
and could derive from an artifact caused by multiple sampling during the experiment if these 153
elements were not homogenously distributed in the sludge.
154
3.2. Particulate and soluble concentrations of elements 155
Soluble concentrations of Cd, Ni, Sb, Se and W were below the method’s limits of detection or 156
quantification (i.e. lower than 12, 721, 102, 1077 and 69 µg/L, respectively) during the whole 157
experiment and the impact of aeration on their distribution cannot be discussed. For the other 158
elements (Figure S4), three different trends were observed. An example of each trend is given in 159
Figure 2. Fe and Mn showed limited variations of their particulate and soluble concentrations 160
during the first 15 days of passive aeration. Then, their soluble concentrations doubled up to the 161
66th day of aeration with a limited influence on their particulate concentration. From the 76th day 162
of passive aeration and during the two weeks of forced aeration, the soluble concentration of Fe 163
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and Mn rapidly doubled. This rapid release in solution generated a slight decrease in particulate 164
Fe (i.e. 4% less) and Mn (i.e. 5% less). Soluble concentrations of Al, Co, Cr, Cu, Mo and Pb were 165
below MLD or MLQ during most of the passive aeration sequence (Figure 2, Figure S4).
166
However, during forced aeration, the soluble concentration of these elements increased above the 167
detection limits and was followed by a decrease of their particulate concentration. In particular, 168
the soluble Mo concentration prevailed in its total content during forced aeration (Figure S4).
169
Finally, As displayed a slightly different behavior. Although its soluble concentration is nearly 170
constant during the first 22 days of passive aeration, a marked increase was observed from day 171
29. This increase is followed by a decrease of its particulate concentration. Unlike other elements, 172
forced aeration had no significant impact on As soluble concentration.
173
174
1600 1650 1700 1750 1800 1850
0 50 100 150 200 250
0 1 2 3 4 5 6 15 22 29 33 43 50 57 66 76 79 82 85 89
mg/Lsludge, particulate mg/Lsludge,soluble, MLQ
Time (days)
Fe
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62 13
175
176
Figure 2. Examples of soluble (dashed line with circles) and particulate (bars) elements' concentration over time. The bold
177
horizontal dashed line is the method limit of detection (MLD) or quantification (MLQ) for soluble elements whereas the vertical
178 dashed line indicates the beginning of forced aeration.
179
Overall, aeration induces a release in solution of all quantified elements (i.e. Al, As, Co, Cr, Cu, 180
Fe, Mn, Mo and Pb). This release was likely caused by direct oxidation of sulfur precipitates in 181
presence of oxygen from the air (Fermoso et al., 2015). However, oxidation performed by 182
indigenous microorganisms such as sulfur oxidizing bacteria (i.e. Acidithiobacillus species) (Jain 183
and Tyagi, 1992) is not excluded, though this hypothesis needs further investigations. In both 184
700 750 800 850 900 950 1000 1050
0 50 100 150 200 250 300 350
0 1 2 3 4 5 6 15 22 29 33 43 50 57 66 76 79 82 85 89
µg/Lsludge, particulate µg/Lsludge,soluble, MLD
Time (days)
Cr
0 1 2 3 4
0 1 2 3 4
0 1 2 3 4 5 6 15 22 29 33 43 50 57 66 76 79 82 85 89 mg/Lsludge, particulate mg/Lsludge,soluble, MLQ
Time (days)
As
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
cases, sulfide oxidation leads to metal sulfide precipitates dissolution (e.g. FeS, CoS, Cu2S, PbS) 185
(Maharaj et al., 2018; Möller and Müller, 2012) as well as the release of sulfate. Indeed, a 186
significant increase of sulfate concentration was measured after the 57th days of passive aeration 187
and during forced aeration (Figure S5). These results are in agreement with the soluble sulfate in 188
sludge suspension found by Carbonell-Barrachina et al. (1999) under oxidizing conditions.
189
Regarding particulate As, it can be hypothesized that it is initially co-precipitated with Fe sulfides 190
(Savage et al., 2000) and consequently released in solution after their dissolution upon oxidation.
191
This is consistent with the slight increase of soluble Fe observed from the 29th day of passive 192
aeration.
193
3.3. DGT-labile elements concentration 194
Labile concentrations of Cd, Cr(III), Cu and Pb were lower than 0.02, 0.2, 2, 0.6 µg/L, 195
respectively, during the whole experiment. The labile concentrations of Mo, Sb and W were close 196
or below the MLDDGT during most of the passive aeration experiment (Figure S5). Labile 197
concentrations of the other elements are given in Figure S5 and typical examples are displayed in 198
Figure 3. Labile Al, As, Co, Fe and Mn rapidly decreased during the first 3-5 days of passive 199
aeration and later their concentration remained rather constant until the 57th day of aeration at 200
least. Conversely, no initial decrease was observed for Ni and Se.
201
Under forced aeration, several elements (i.e. Al, Mo, Ni, Sb, Se and W) displayed a rapid 202
increase of their DGT-labile concentrations followed by a decrease, except for Mo and W. As and 203
Co slightly decreased immediately after forced aeration and their concentration increased again at 204
the 85th day. After 57 days of passive aeration Fe and Mn behavior differs from the other 205
elements since their labile concentrations continued to decrease, even under forced aeration.
206
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62 15
The decrease of labile Al at the beginning of passive aeration may be explained by the presence 207
of negatively charged hydroxide complexes (e.g. Al(OH)4-) at pH 7.8 ± 0.3 that are not efficiently 208
sampled by Chelex-DGT (Panther et al., 2012). The increase of labile Al, As, Co, Ni after 57 209
days of aeration could be a direct consequence of their release form sulfide species as discussed 210
in 3.2. In contrast, the decrease of Fe and Mn labile concentration is not associated with the 211
increase of their soluble fraction, especially at the end of the forced aeration, meaning that part of 212
these soluble elements are DGT-inert (e.g. colloids such as Fe(II)-phosphate or strongly 213
complexed with organic functional groups such as thiol groups (Shakeri Yekta et al., 2014)).
214
Therefore, it can be concluded that oxidation converts a part of labile species of Fe and Mn into 215
soluble non-labile species. Similarly, Øygard et al. (2007) showed a strong decrease of labile Fe 216
(determined with cation exchange SPE cartridge) during the exposition of leachate to oxygen, 217
while particulate and colloidal Fe (e.g. iron oxides) increased.
218
Conversely, the delay observed for the increase of labile As and Co concentration during forced 219
aeration let suppose slow mechanisms of conversion into labile form. Moreover, adsorption onto 220
Fe/Mn colloids could have occurred.
221
222
0 2 4 6 8 10 12 14 16 18
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 CDGT(µg/L)
Time (days)
Al
0 2 4 6 8 10 12
0 1 2 3 4 5 6
CDGT(µg/L)
Time (days)
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
223
224
225
Figure 3. Examples of labile elements' concentration over time. The bold horizontal dashed line is the MLQDGT whereas the
226
vertical dashed line indicates the beginning of forced aeration. The inset is an enlargement of the first 6 days of the experiment.
227
0 20 40 60 80 100
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 CDGT(µg/L)
Time (days)
As
0 20 40 60
0 1 2 3 4 5 6
CDGT(µg/L)
Time (days)
0 1000 2000 3000 4000 5000
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 CDGT(µg/L)
Time (days)
Fe
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
0 1 2 3 4 5 6
CDGT(µg/L)
Time (days)
0 1 2 3 4 5
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 CDGT(µg/L)
Time (days)
Ni
0 1 2 3
0 1 2 3 4 5 6
CDGT(µg/L)
Time (days)
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
62 17
3.4. Environmental impact of digestate aeration 228
In this study, performed at laboratory-scale in controlled conditions, it was reported that aeration 229
regime modifies TEs distribution among labile, soluble and particulate fractions. It is assumed 230
that the observed TEs’ fractionation can help to anticipate phenomena related to air exposure 231
occurring on field during digestate management. Indeed, the passive aeration experiment could 232
show the phenomena that can be expected during the storage of digestate in open tanks. Usually, 233
the required storage time of digestate before land spreading may range from 90 days to 10 234
months depending on the country and digestate spreading season (Plana and Noche, 2016). The 235
variation on TEs’ mobility observed during forced aeration is hypothesized to be similar to the 236
one occurring during digestate handling for land application since the contact between air and 237
digestate is significant.
238
Passive and forced aeration resulted both in a release in solution of Al, As, Co, Cr, Cu, Fe, Mn, 239
Mo and Pb. Therefore, aeration of digestate could increase mobility of TEs over time. Under 240
passive aeration, dissolution was slow during the first four weeks. Consequently, storage of 241
digestate in an open tank could increase only marginally TEs mobility provided the storage 242
duration is limited. However, dissolution increased significantly after approximately 30 days of 243
passive aeration for most elements. Such increase is likely correlated to the increase of the 244
surface to volume ratio (from 0.39 dm-1 to 0.45 dm-1 after 30 days of aeration) that controlled the 245
rate of aeration of the digestate during the experiment. Therefore, design of digestate storage tank 246
would be an important parameter to limit the increase of trace element mobility during storage. In 247
this context, digestate storage tank with low surface to volume ratio (i.e. important height) should 248
be favored. Forced aeration resulted in an important dissolution of all the quantified elements, 249
except for As. Therefore, it is assumed that TEs’ mobility could be strongly increased during 250
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digestate handling for land spreading. A “safety factor” which counts for TEs’ oxidation during 251
digestate handling should be considered for environmental risk assessment.
252
Alongside particulate/soluble fractions, DGT-labile elements were measured during this study.
253
DGT-labile species (i.e. free + weak complexes) are the most reactive species and would 254
represent the most readily bio-accessible fraction of TEs (Zhang and Davison, 2015). During 255
passive aeration, although soluble elements’ concentration increased, no correlated increase of 256
DGT-labile concentrations was found for Al, As, Co, Fe, Mn, and Se. Only DGT-labile Ni 257
showed a small delayed increase (≥ 60 days, within a factor 2). Therefore, storage of digestate in 258
an open tank could have no impact on the labile fraction of most of the studied TEs.
259
During forced aeration, except for Fe and Mn, all quantified labile elements rapidly increased.
260
Moreover, the bio-accessibility of labile elements could increase after land application depending 261
on the soils’ sorption capacity (Dragicevic et al., 2018b; Kabata-Pendias, 2004) and plants uptake 262
mechanisms (Lehto et al., 2006; Tack, 2010). Such hypothesis should be further studied for risk 263
assessment. It was also observed that labile Al, As, Co, Ni, Sb and Se decreased after one week 264
of forced aeration, therefore, it is not excluded that their bio-accessibility could remain unaltered 265
during digestate land application.
266
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62 19
4. Conclusions
267
In this work, the influence of aeration of sewage sludge digestate on the fractionation of fourteen 268
TEs (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se and W) was studied with a laboratory- 269
scale tank. Aeration promoted dissolution of all the quantified elements (i.e. Al, As, Co, Cr, Cu, 270
Fe, Mn, Mo and Pb), which was certainly due to oxidation of metal sulfide precipitates.
271
Therefore, it was assumed that the observed increase of TEs mobility due to aeration may likely 272
occur during storage in open tank or digestate handling before land application. However, this 273
dissolution did not promote an increase of DGT-labile concentrations during passive aeration.
274
Conversely, forced aeration promoted an increase of the labile Al, As, Co, Mo, Ni, Sb, Se and W.
275
Therefore, it can be assumed that passive aeration of digestate like in open storage tank would not 276
increase TEs bio-accessibility unless significant aeration such as during digestate handling for 277
land spreading takes place.
278
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Conflict of interest
279
The authors declare no conflict of interest.
280
Acknowledgements
281
Patrice Fondanèche is acknowledged for his assistance in the laboratory, especially during ICP- 282
MS and MP-AES analyses.
283
Funding sources
284
This work was supported by the European Union’s Horizon 2020 research and innovation 285
programme under the Marie Sklodowska-Curie grant agreement No 643071.
286
Appendix A. Supporting information
287
The supporting information is available at the following link (to be mentioned).
288
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62 21
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